1. Field Of the Invention:
This invention relates generally to a combined sea water desalinization and electrical power generation system and, more particularly, such a system which operates with greater efficiency and high salts recovery, with little or no discharge of concentrated brine material.
2. Description of the Related Art:
The need for economical desalination plant has grown as reliable supplies of fresh water have diminished in traditionally water-short areas of the world and in semi-arid regions experiencing rapid population growth. The major desalination processes use in such plants today can be broadly classified as either thermal or membrane processes.
Thermal processes are based on the distillation of salt water wherein salt water is boiled, and the steam evolved therefrom is collected and condensed into desalinated water. The most widely used thermal process uses multistage flash distillation, also known as MSF, which is based on the principle that water will boil at lower temperatures when it is subjected to lower pressures. Using MSF, heated salt water is fed into a flash chamber in which the pressure is gradually lowered which allows the salt water to boil at lower temperatures. The vapor produced is condensed on tubes that carry fresh, cool salt water into the plant. In the heat-exchange process, steam heats the cooler salt water, while the vapor condenses into desalinated water. The higher concentrated, unevaporated sea water is then delivered to a second chamber maintained at a lower pressure, where the process is repeated. For large desalination plants, a large number of flash chambers may be used.
Most membrane plants are based on reverse osmosis processes wherein saline water is pumped to a pressure above its osmotic pressure. The compressed saline water is ultra-filtered by a semi-permeable membrane which allows water molecules to pass through while preventing passage of salt molecules. Fresh water is then collected from the other side of the membrane.
One major constraint in all desalination plants is undesirable salt formation on various surfaces on the machinery. For example, calcium sulphate salts precipitation, which can not be prevented by pH control, will limit the maximum boiling temperature of sea water to 120 degrees C. This is mainly due to the salt material being deposited on heat transfer surfaces located in the evaporators which increases the resistance for heat transfer. This, in turn, increases the energy requirement for the plant.
Combined desalination and power generation plants, also known as dual purpose plants, are commercially used today. In general, these plants utilize the expanded steam from the power plants' turbines to supply thermal energy used in the desalination process. Currently, research is directed towards improving the temperature limitations of the plants by chemical pre-treatment of the salt water. One alternative approach has been to use direct contact heat transfer equipment which eliminates the need for heat transfer surfaces. With direct contact heat transfer processes, a heat transfer medium, such as hydrocarbon oils, is used to transfer heat to the salt water. Unfortunately, problems with emulsion formation, oil-water separation, and oil degradation have limited the development of these processes.
Another approach to resolving the temperature limitations of these plants was described by Blaskowski in U.S. Pat. No. 3,352,107. In Blaskowski, superheated steam is used as a heat transfer medium in a combined power generating-desalination plant. Unfortunately, the system was not commercially developed due to important thermodynamic and design considerations.
Another desirable goal for most desalination plants is the total recovery of various salts from the sea water. Unfortunately, due to inherent thermal inefficiencies in the plant's designs, recovery of salts from current desalination plants have not be commercially developed.
More efficient dual purpose plants are needed today. In addition, such plants which allow for greater salt recovery and that substantially reduce or eliminate the concentrated brine therefrom, would be highly desirable.